A tire for a passenger car generally comprises a radial carcass reinforcement anchored in two beads and surmounted radially by a crown reinforcement, itself surmounted radially by a tread designed to be in contact with the road during rolling. The crown reinforcement comprises at least two layers, each formed of a plurality of reinforcing elements, the reinforcing elements of one layer being crossed relative to those of the adjacent layers.
These layers can for example be made in the form of plies, each ply consisting of a plurality of filaments or cords essentially parallel to one another. In another known form, the reinforcing elements are positioned individually or in groups of several at a time during the fabrication of the tire, without going through the prior stage of producing the plies. For convenience, in the text that follows, “ply” will be used to denote any layer of the crown reinforcement, regardless of how the said layer is obtained.
In the case of a tire whose carcass has textile reinforcing elements, to increase the mechanical triangulation effect in combination with the working plies it is known to provide the crown of the tire with a single additional ply whose reinforcing elements make an angle longer than 60° relative to the equatorial plane of the tire.
By definition, the equatorial plane of a tire is the plane perpendicular to the tire's rotation axis, which divides the crown reinforcement axially into two essentially identical parts.
In another known arrangement, instead of a single ply two additional plies can be used, positioned on either side of the equatorial plane whether symmetrically or not relative thereto.
Besides, and having regard to the high rotation speeds to which passenger car tires are subjected, it is known to provide the crowns of such tires with at least one additional reinforcement whose reinforcing elements are arranged essentially parallel to the equatorial plane (i.e. making an angle smaller than 10° with that plane). The reinforcing elements of this additional reinforcement can be textile or metallic and are chosen to have appropriate resistance to the forces to which the tires are subjected in service.
In the state of the prior art, Japanese document JP06/171308 shows a tire crown comprising two working plies, an additional meridian reinforcing ply (i.e. a ply whose reinforcing elements make an angle larger than 60° with the circumferential direction) positioned radially between the carcass and a first working ply, and radially outside the working plies, two additional layers of reinforcement elements orientated in the circumferential direction. On the other hand, document JP05/069702 shows a tire that can have a meridian reinforcement ply centered on the equatorial plane, or two meridian reinforcement plies arranged symmetrically relative to the said plane; that document also specifies the presence of two circumferential reinforcements arranged at the axially outer ends of the meridian reinforcement ply.
Another document, FR 2421074, describes a tire comprising two small meridian reinforcement plies arranged symmetrically relative to the equatorial plane and under the ends of the working plies; a single circumferential reinforcement is also provided, which is positioned axially between the said two small meridian reinforcement plies.
It was found that these solutions were not entirely satisfactory during the use of tires that have to meet the performance requirements of the vehicles produced nowadays and the requirements of users in relation to wear resistance (increased useful life) of the tires in service.